It is well known in the silver halide photography that incident light used for imagewise exposure of a radiation-sensitive silver halide emulsion layer is only partially absorbed within that layer. If exposing light that has passed through the emulsion layer is reflected back, the reflected light, to the extent that it is absorbed during its second traverse, acts to increase the effective speed of the silver halide emulsion layer. Unfortunately, reflected light is known to reduce greatly image sharpness. In fact, the term "halation" was coined early in photography to describe this effect, since a bright object that is photographed without protection against reflected light appears to be surrounded by a halo, attributable to scattered reflected light. To avoid halation, it is a common practice to coat beneath the radiation-sensitive silver halide emulsion layer or layers of a photographic element an antihalation layer. The function of the antihalation layer is absorb light that has passed through the overlying emulsion layer or layers during imagewise exposure. Antihalation is described by Keller Science and Technology of Photography, VCH, New York, 1993, 3.2.16. Antihalation Coatings, pp. 68 and 69.
Kofron et al U.S. Pat. No. 4,439,520 demonstrates that the image sharpness of an underlying silver halide emulsion layer is increased when an overlying radiation-sensitive silver halide emulsion layer is selected to contain a high (&gt;8) aspect ratio tabular grain emulsion.
The use of silver halide epitaxy to increase the sensitivity of high aspect ratio tabular grain emulsions was first disclosed by Maskasky U.S. Pat. No. 4,435,501. Other illustrations of composite grains formed by epitaxial deposition are provided by Research Disclosure, Vol. 389, September 1996, Item 38957, I. Emulsion grains and their preparation, A. Grain halide compositions, paragraph (7). More recent illustrations of silver halide epitaxy applied to tabular host grains are provided by Daubendiek et al U.S. Pat. Nos. 5,494,739, 5,503,971 and 5,576,168, Deaton et al U.S. Pat. No. 5,582,965, Eshelman et al U.S. Pat. Nos. 5,612,175, 5,612,176 and 5,614,359, and Levy U.S. Pat. No. 5,612,177.